Abstract: We present HipHop.js, a synchronous reactive language that adds synchronous concurrency and preemption to JavaScript. Inspired from Esterel, HipHop.js simplifies the programming of non-trivial temporal behaviors as found in complex web interfaces or IoT controllers and the cooperation between synchronous and asynchronous activities. HipHop.js is compiled into plain sequential JavaScript and executes on unmodified runtime environments. We use three examples to present and discuss HipHop.js: a simple web login form to introduce the language and show how it differs from JavaScript, and two real life examples, a medical prescription pillbox and an interactive music system that show why concurrency and preemption help programming such temporal applications.